Asterisk - An open source telephony switching and private branch exchange service for Linux.
RingCentral - RingCentral is the leading provider of cloud-based communications and collaboration solutions for small business and enterprise companies
FreeSWITCH - FreeSWITCH is an open-source media application designed to support popular protools such as SIP and WebRTC and provides a platform to develop voice and video applications.
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
Wazo.io - The UCaaS Platform Built for MSPs
Phonebooth - Phonebooth is one of the leading voip providers, providing phone systems & phone service for small businesses.